Chapter 13 Configuring VLANs

Configuring VLAN Trunks

Figure 13-2shows a network of switches that are connected by ISL trunks.

Figure 13-2 Switches in an ISL Trunking Environment

Catalyst 6500 series

switch

ISLISL

trunk trunk

Switch

Switch

VLAN1VLAN3

ISL

ISL

trunk

trunk

 

Switch

Switch

 

 

VLAN2

VLAN2

VLAN1

VLAN3

45828

You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more information about EtherChannel, see Chapter 37, “Configuring EtherChannels and Link-State Tracking.”

Ethernet trunk interfaces support different trunking modes (see Table 13-4). You can set an interface as trunking or nontrunking or to negotiate trunking with the neighboring interface. To autonegotiate trunking, the interfaces must be in the same VTP domain.

Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-Point Protocol. However, some internetworking devices might forward DTP frames improperly, which could cause misconfigurations.

To avoid this, you should configure interfaces connected to devices that do not support DTP to not forward DTP frames, that is, to turn off DTP.

If you do not intend to trunk across those links, use the switchport mode access interface configuration command to disable trunking.

To enable trunking to a device that does not support DTP, use the switchport mode trunk and switchport nonegotiate interface configuration commands to cause the interface to become a trunk but to not generate DTP frames. Use the switchport trunk encapsulation isl or switchport trunk encapsulation dot1q interface to select the encapsulation type on the trunk port.

You can also specify on DTP interfaces whether the trunk uses ISL or IEEE 802.1Q encapsulation or if the encapsulation type is autonegotiated. The DTP supports autonegotiation of both ISL and

IEEE 802.1Q trunks.

Note DTP is not supported on private-VLAN ports or tunnel ports.

 

 

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

 

 

 

 

 

 

OL-9775-02

 

 

13-17

 

 

 

 

 

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Cisco Systems 3750E manual 13-17, Switches in an ISL Trunking Environment